GL_MESA_EGL_sync Extension Proposed For The Khronos OpenGL Registry
EGL_KHR_fence_sync adds the concept of synchronization objects to EGL as sync primitives and similar to GL_ARB_sync. But with GL_MESA_EGL_sync, the desire is to be more like EGL_KHR_fence_sync rather than the ARB_sync extension due to some platform extensions in turn relying on the EGL extension.
One of the main motivators for MESA_EGL_sync in OpenGL appears to be for enabling EGL_ANDROID_native_fence_sync support and using Linux sync file objects for synchronization between processes using OpenGL and for explicit fencing with the Linux DRM/KMS code.
There's been work-in-progress Mesa code under review while the extension specification is pending review for the OpenGL Registry.
This is just one of several MESA-developed/originating extensions in recent years.